Feature/auto refresh files individually

[abelsromero]

Related to #472

Make it so that auto-refresh mojo only converts modified or newly created files. This is far more efficient and more importantly, will offer for a much responsive experience for users.

This PR contains:

• Refactor of AsciidoctorMojo so all constructions logic can be applied to individual files.
• Added message and skip logic when no sources are found. This is not treated as error and allows finishing sooner, so may help usign having problems with configurations.
• Modified messages in RefreshMojo:
  • "File ..." has been changed to "Source file ..." to better distinguish between AsciiDoc sources and resources.
  • "Re-converted" has been removed since the reuse of the AsciidoctorMojo already shows "Converted ..." message
  • "doc" has been replaced by "document"

PS: Still need to see how to deal with deleted files. That includes renaming a file, which is triggered as a creation + deletion.

[coveralls]

Coverage increased (+0.6%) to 79.476% when pulling 0987c07 on abelsromero:feature/auto-refresh-files-individually into 10b2899 on asciidoctor:master.